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Comment by nullc | Hacker News Reader

Your bog standard old AC clock is synchronously timed from the input AC.

So one just need synthesize a perfect 60Hz ~115vac input and the clock will keep perfect time. (In theory the grid tries to keep somewhat accurate time, but in practice at least on the west coast the departures can be pretty substantial.)

I have a wall clock here that keeps coordinated Mars time, driven off a local atomic clock.

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> (In theory the grid tries to keep somewhat accurate time, but in practice at least on the west coast the departures can be pretty substantial.)

I was under the impression that even though the frequency drifts can be substantial, the grids compensate so that the total number of cycles over a longer time matches pretty exactly what the target frequency would have given?

I.e. if the target is 60 Hz, and 61 Hz happens for an hour, then the grid will later on intentionally go down to 59 Hz for an hour (or, perhaps more likely, a frequency close to, but below, 60 Hz for a longer time) to compensate. This is an easier task than targetting a fixed frequency, because the total number of cycles is just a matter of counting and crude intentional drifts for compensation. And it's the total number of cycles that AC-synced clocks use, so they might run slow or fast for a short while, but will then correct themselves by running faster or slower later on.

Can someone more knowledgeable comment?

Your understanding isn't incorrect but you're probably expecting smaller time and error scales then there are in practice:

Here are some charts of grid time offset from 2014 (I think from Menlo Park): http://users.megapathdsl.net/~hmurray/time-nuts/line/Calif-6... -- so you can see this spanning 60 seconds in a year.

[Source post: https://www.febo.com/pipermail/time-nuts/2015-July/092791.ht... ]

Here is some more recent data: http://users.megapathdsl.net/~hmurray/time-nuts/60Hz/60Hz-20...

Interesting! Thanks!

this device was not connected to AC input. And, btw, few yrs ago in EU AC based clocks had an issue due to problems in the grid (frequency was < 50Hz)

https://www.theguardian.com/world/2018/mar/08/european-clock...

I use ntp and this: https://www.npl.co.uk/msf-signal

I set my wrist watch manually. That's my time of first resort. It's slightly inaccurate but very, very reliable and handy (by definition!) I bought it about 15 years ago and its solar powered. Never failed yet. My phone keeps decent time but the battery needs charging - fails very infrequently but needs waking up. My laptop keeps a decent tick. It has a fairly decent local oscillator and ntp with several sources. Similar probs to phone and takes a while to boot. My three GPS synched ntp stratum 1 servers (RPis) that I keep in the office keep a very good tick.

For my needs, consistency is more important than real accuracy but I do like to see sub millisecond accuracy available. It makes log analysis a lot easier.

I was under the impression that if your frequency is not exactly 60 Hz then a place where two separate power sources meet could result in what you might describe as a kaboom. Is that not correct?

Power sources are not synchronized to a fixed 60 Hz, but to each other. If the frequency drifts off 60 Hz, the entire grid drifts together.

There is a physical reason for that - all the generators connected to the grid, including massive nuclear/coal-steam-turbine powered ones, literally slow down due to increased load. Thats why in AC grid actually the frequency decreases and not the voltage (though voltage may drop also because of higher currents in wires). I'm generally curious how battery-backed sources or solar plants respond to that, since their output frequency is artificial due to DC-AC inverters being used. Do they adapt to the grid frequency following some kind of control mechanism, or do they literally experience higher load when out of sync?

Inverter sources also sync to the grid.

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